The present invention relates generally to an apparatus suitable for the detection of particle motion waves. In another aspect, the present invention pertains to an apparatus suitable for the detection of seismic waves, for example, those seismic waves studied by the geophysical industry in determining subsurface geophysical characteristics.
In the field of seismic exploration, it is necessary to detect components of ground motion along each of the coordinate axes in order to correlate detected vibrations to vibrations characteristic of pressure and shear waves. Traditionally, this is done by orienting seismometers so that a principal axis of sensitivity of each seismometer is aligned as closely as possible with a direction of suspected maximum particle displacement.
The vibrations and maximum particle displacements associated with pressure or P-Waves, are perpendicular to the wave front, or parallel to the direction of propagation of the pressure wave. The vibrations and maximum particle displacements associated with shear or S-Waves, are in a plane tangent to the shear wave front, i.e. transverse. Since a shear wave produces transverse particle motion, it can exhibit polarization effects, i.e. it may be composed of two prependicular components. In the conventional nomenclature shear waves with particle motion only in a horizontal plane are called SH-Waves, and shear waves with a component of particle motion in a vertical plane are called SV-Waves. SV-Waves will usually have a component of horizontal particle motion, but SV particle motion can be considered to be constrained to be in a vertical plane defined by the ray paths from source to reflector and from the reflector to the receiver. At an interface between two media with different acoustic impedances P-Waves and SV-Waves can be converted into each other by non-normal incidence to the interface. SH-Waves are not converted to SV-Waves or P-Waves at interfaces.
P, SV and SH waves, like other types of seismic waves, propagate in a direction generally away from a seismic event or shot point. The exact direction of propagation may change due to reflection refraction of the waves from formations with which the waves have come into contact during the course of propagation.
To detect P, SV and SH waves, the method utilized by the prior art was thus orienting a first geophone with its principle axis of sensitivity aimed generally vertically, so as to detect predominately P-Waves reflected from deep events and traveling nearly vertically at the surface; orienting a second geophone with its principal axis of sensitivity generally horizontally and in line with the source to receiver axis, so as to detect predominately SV-Waves; and orienting a third geophone with its principal axis of sensitivity generally horizontally and perpendicular to the plane defined by the axes of the first and second geophones, so as to detect predominately SH-Waves. Utilization of this method requires two geophones designed for horizontal operation and one geophone designed for vertical operation.
A geophone is basically a device which translates a mechanical vibration into an electrical signal which duplicates the character of the mechanical vibration. Geophones transform mechanical energy into electrical energy by utilization of a coil and magnet arrangement. A magnet is mounted in a fixed position on a frame which is in turn secured to the earth so as to vibrate therewith. A coil surrounds the magnet. The coil is movably mounted to the frame such as by springs. When vibrations of the earth produce movement in the frame and magnet, the coil, because of inertia, tends to remain in the same position. The relative movement between the coil and magnet along a longitudinal axis of the assembly induces an electrical signal in the coil which is proportional to the velocity of the coil relative to the magnet. The principal axis of sensitivity is along the direction the maximum displacement between the coil and magnet.
In most applications, the coil is mounted around the magnet so as to allow movement only along the longitudinal axis of the coil. The mounting is designed so as to be only minimally effected by gravitational forces. Gravitational forces acting on movement of the coil along its longitudinal axis will vary with the sine of the angle which the longitudinal axis makes with the horizontal. Because the gravitational forces which must be neutralized vary with the sine of the angle at which the geophone is to be operated, the design of the coil mounting must be different for geophones designed to operate at different angles in order to maintain gravitational distortion at a minimum. This is commonly done by mounting geophone coils to be used at different angles of operation on springs of different strengths.
Utilization of different strength springs has an effect on relative movement which occurs between the coil and the magnet when a mechanical vibration is received. This effect is manifested as a change in the velocity, magnitude and/or frequency of the movement between the coil and magnet, and thus results in a change in the nature of the electrical signal produced. Utilization in the same detector of geophones designed to operate at different angles, as in prior art, required careful matching and calibration of the geophones used, to insure that each geophone would produce an equivalent signal for equivalent mechanical vibrations received along the longitudinal axis of each coil. It is thus extremely desirable to avoid using geophones designed to operate at different angles and having springs of different strengths in the same detector.
As previously mentioned, the effect of gravity on relative movement between the geophone coil and magnet is proportional to the sine of the angle which the longitudinal axis of movement makes with the horizontal. Leveling of horizontally mounted geophones is especially critical, due to the fact that gravitational forces and geophone motions are at right angles. If a detector comprised of horizontally mounted geophones is improperly leveled in affixing it to the earth, or if it becomes partially dislodged when receiving mechanical vibrations, the horizontally mounted geophones may be disoriented sufficiently from the horizontal so that gravitational forces along the longitudinal axis of movement act to produce a distorted signal which is unrepresentative of the received mechanical vibration. It is thus desirable to utilize a detector which does not utilize horizontally oriented geophones.
The moment of inertia of a seismic detector about the point at which it is affixed to the earth and also about its central axis of desirably of small magnitude. When the detector has a low moment of inertia about the point at which it is affixed to the earth, it accelerates more readily about the point at which it is affixed when disturbed by a seismic wave, and decelerates more readily when the disturbance has ended, than a detector with a higher moment of inertia. It thus more closely follows the movements of the earth and gives more accurate results attributable at least in part to the elimination of spurious housing motion caused in part by harmonic vibrations. Additionally, because it develops less torque about the point at which it is affixed to the earth once set into motion, it is less likely to become dislodged or shift as seismic waves pass under it than a detector with a higher moment of inertia. It is thus desirable to keep the mass of the detector low, and the distribution of the mass close about the point at which it is to be affixed and about its central axis.
In-field usage of geophones is characterized by rough handling. It is thus desirable to protect the geophones to prevent damage which may be difficult to repair in the field. Protective devices, such as geophone housings, must be sturdy enough to protect the geophones contained within and yet light enough to accurately follow the movements of the earth so as to provide a representative signal of the vibrations received.